The present invention generally relates to a method of forming a shaped image in a workpiece. More specifically, the present invention relates to a method of forming a shaped image in a workpiece using a high energy source and a layer disposed proximate the workpiece such that the layer prevents debris from the workpiece from dispersing away from the workpiece.
Techniques for forming a shaped image in a workpiece are many. Such techniques are widely used in the manufacture of many types of electronic devices such as magnetic dam storage disks with optical servo tracks, memory card circuits, and flexible circuits. Related techniques are also employed to mark various devices with information such as bar-codes, to create printing elements, such as lithographic plates, and to generate ornamental designs.
Stamping is one technique for creating a shaped image in a workpiece. For example, presses with stamping dies create optically readable servo stitches in magnetic data storage disks. One problem with the stamping technique is that stamping dies have relatively short life spans. Also, the elastic nature of the disks causes changes in the geometry of stamped stitches over time.
Chemical etching is another example of a technique for creating a shaped image in a workpiece. In this technique, photoresist is applied to a substrate and patterned in a known manner. Developed portions of the resist are then removed by chemical etching to leave the shaped image. The chemicals which perform the etch are not entirely beneficial. For instance, the chemicals tend to undercut undeveloped portions of the workpiece. This undercutting limits the size and location of the shaped image which may be formed in the workpiece.
Other well-known processes for creating a shaped image in a workpiece include electron-beam, ion beam, corona, and plasma treatment. These methods are either continuous or long pulse length etching processes which, due to their low energy flux, yield a low heat transfer rate. The low heat transfer rates are detrimental when etching surface coatings such as polymer-based coatings. Specifically, the low heat transfer rates create an undesirable thermal treatment effect in areas of the coating other than the etched areas.
Laser-based techniques are also useful for creating a shaped image in a workpiece. One technique utilizes an Argon/Ion laser to directly bum optically-readable servo stitches one by one into a magnetic dam storage disk. The laser beam is optically switched on and off while the disk is spinning and a final lens objective is translated. U.S. Pat. No. 4,323,755 to Nierenberg concerns a method of producing a machine readable coded marking in a surface of a workpiece, such as a glass faceplate panel of a television picture tube, by vaporizing parallel areas of similar width in the panel surface using a CO.sub.2 laser. U.S. Pat. No. 4,515,867 to Bleacher et al. illustrates a technique for directly marking a glass funnel of a television picture tube by ablating image features into a pigmented inorganic coating placed on the funnel. U. Sowado, H. J. Kahlert & D. Basting, in "Excimer Laser Processing of Thin Metallic Films On Dielectric Substrates," 801 High Power Lasers 163-167 (1987), comment upon patterning metal coatings of polymer substrates using ablation.
U.S. Pat. No. 5,204,517 to Cates et al. discloses a method of removing paint coatings from metal and polymer substrates using an excimer laser. The laser has a relatively long pulse width on the order of 0.2 microseconds during which the energy density is in the range of 1-5 J/cm.sup.2. The method involves control of a paint removal process by monitoring spectral emissions of the paint coating.
U.S. Pat. No. 5,061,604 to Ouderkirk et al. describes irradiation of a surface layer of semi-crystalline polymer with an excimer laser to create an imagewise distribution of quasi-amorphous polymer within the surface layer has been mentioned. A reactive ion etching process is then utilized to preferentially remove the semi-crystalline polymer after irradiation of the surface layer.
U.S. Pat. No. 4,822,451 to Ouderkirk et al., U.S. Pat. No. 4,868,006 to Yorkgitis et al, and U.S. Pat. No. 4,879,176 to Ouderkirk et al. also concern irradiation of a surface layer of semi-crystalline polymer with an excimer laser to render portions of the surface layer quasi-amorphous. It has been noted that the presence of the quasi-amorphous layer tends to enhance bonding of the semi-crystalline polymer to other materials generally, including adhesive materials. It has also been noted that the presence of the quasi-amorphous layer reduces optical reflectance and increases optical transmission of the semi-crystalline polymer, increases coating adhesion to the semi-crystalline polymer, and reduces the coefficient of friction of the surface of the semi-crystalline polymer.
Such direct laser formation of individual shaped image features may be desirable for some workpieces with small numbers of image features and for some projects with relatively small numbers of workpieces. However, direct laser formation of individual features is not entirely without problems. For example, ablation typically yields high energy fragments of debris which often splash onto optical equipment associated with the laser. Cleaning the debris fragments from the optical equipment is disruptive and impractical in industrial applications.
One potential solution to the debris problem involves moving the final optical surfaces some added distance away from the workpiece. However, this solution is undesirable for a variety of reasons. For instance, laser beam control and orientation relative to the workpiece is more technically challenging and less economically efficient when the optical surfaces are moved further from the workpiece. Additionally, space considerations sometimes prevent movement of the optical surfaces away from the workpiece.
U.S. Pat. No. 4,032,743 to Erbach et al. discloses a rotating cylindrical drum and a plurality of stationary lasers for boring closely spaced holes through foil strips mounted on the drum using a single pulse of a laser for each hole. A strip of film is connected to fixed-rate supply and take-up reels and is positioned between the foil and a lens of the laser to protect the lens from vaporized material. The film is transparent to the radiation wavelength of the laser.
Another consideration is that, though direct formation of individual image features is sometimes beneficial for workpieces with fewer image features and for smaller batches of workpieces, direct feature formation is not always an optimum choice. For example, direct laser formation of image features, one at a time, requires much more time than if the laser operated on multiple images or image features arranged about the workpiece.
Technological advances have been developed which allow laser operation on more than one image or image feature at a time. For example, U.S. Pat. No. 4,877,480 to Das discloses a contact lithographic technique for forming shaped images in a workpiece, such as an alumina-coated ceramic substrate. According to Das, a mask of material that is highly reflective in the wavelengths of the selected laser is placed in contact with the alumina coating. Radiation from a CO.sub.2 laser is applied to the mask to remove portions of the alumina coating which are not masked. The reflective surface of the mask reflects the laser radiation away from areas of the workpiece covered by the mask.
M. Gauthier, R. Bourret, E. Adler, & Cheng-Kuei Jen, in "Excimer Laser Thin Metallic film Patterning On Polyvinyledene Difluoride," 129 Materials Research Society Symposium Proceedings , 399-404 (1989), discusses a technique for ablating metal coatings located on the front and back sides of a polymer film using an excimer laser and a mask to form patterns in each of the metal coatings. The laser first ablates a pattern in the coating on the front side in a single pulse. The laser then ablates a pattern in the coating on the rear side in a single pulse by passing radiation through the front side of the polymer film toward the coating on the rear side.
A mask which is placed in relation with a workpiece, as in the Das patent, may be a supported or an unsupported mask. An unsupported mask is a mask which does not include underlying mechanical support for transparent window portions of the mask which shape the areas to be removed from the workpiece. The use of unsupported masks may be disadvantageous for many reasons. For example, unsupported masks tend to absorb heat which may cause mask shape distortion. Also, due to the inability of an unsupported mask to support isolated areas of mask material, it is not possible to make certain image features, such as an X-Y pattern or the center of the letter "O", using an unsupported mask. It is also difficult under some manufacturing tolerance extremes to maintain an unsupported mask in intimate contact with the workpiece being imaged. This may impair resolution and create alignment problems.
A supported contact mask includes mechanical support for all areas of the mask and avoids many problems associated with unsupported masks. However, it has been found that even the use of a supported mask in combination with a laser is not entirely satisfactory for creating patterned images in workpieces. One major problem is fragment debris created by imaging processes, such as ablation. Specifically, debris often splashes onto the mask. Cleaning the mask after each use is not practical in industrial applications. Also, debris may cause spots in transparent window areas of the mask during the imaging process. Such spots cause diffraction of the laser beam and may prevent accurate image feature creation.
One potential solution to the debris problem is to move the mask away from the workpiece. In some applications, projection lithography incorporates a mask positioned away from the workpiece. J. R. Lankard & G. Wolbold, in "Excimer Laser Ablation of Polyimide in a Manufacturing Facility," 54 Applied Physics A--Solids and Surfaces, 355-359 (1987), discuss application of projection lithography in connection with laser ablation of polymer-coated substrates.
Projection lithography has a number of limitations, though, including high equipment costs and low laser beam throughput. Also, projection lithography may only expose small workpiece areas at a time. This small field size complicates imaging of large area, non-repeating shaped images since the mask must be repeatedly and precisely moved in relation to the imaged areas. Movement of the mask away from the workpiece may also decrease the resolution of image features in the workpiece and may cause image blooming between the mask pattern and the workpiece.